[llvm-dev] Cross compiling for Baremetal ARM without using GCC

Wed Nov 1 12:49:24 PDT 2017

Dear Peter,

Thank you very much for the prompt response.
It's good news that you managed to build compiler_rt, but when I followed the same instruction, I met some issues like
"Specified distribution component 'builtins-armv6m-none-eabi' doesn't have a Target".
I have a lot of questions but I think I need to try the instruction once more and get back to you.

Have a good day!

Best Regards,
Daniel

> -----Original Message-----
> From: Peter Smith [mailto:peter.smith at linaro.org]
> Sent: Wednesday, November 01, 2017 5:02 AM
> To: Daniel Kang <Daniel.Kang at cypress.com>
> Cc: llvm-dev at lists.llvm.org; Nick Forrette <nick.forrette at cypress.com>; Mike
> Noel <mike.noel at cypress.com>
> Subject: Re: [llvm-dev] Cross compiling for Baremetal ARM without using GCC
>
> Hello Daniel,
>
> I don't have full answers but I may be able to move you slightly forward. I'm
> assuming that you want an answer that involves only open source tools.
>
> I've managed to successfully build compiler-rt using Jonathan Roelofs
> BaremetalARM.cmake in clang/cmake/caches and an arm-none-eabi gcc
> installation as sysroot. One thing that I needed to do for it to work was to
> move compiler-rt from llvm/projects/compiler-rt to llvm/runtimes/compiler-
> rt. I haven't got any experience at building newlib or Musl with clang instead
> of gcc.
>
> I've been working on Arm support in LLD, in theory most of the parts are
> there [*] and the linker script support has come a long way recently.
> However I've not actively tested on embedded systems yet as my focus has
> been on Linux/BSD like targets. At least a few people have had problems
> with LLD always allocating the ELF header and Program Header in the first
> PT_LOAD Program Header, which is definitely not what you want for an
> embedded system. If you do run into problems please let us know and raise
> bug reports if possible.
>
> Peter
>
> [*] There aren't any range extension thunks that work for Cortex-M0, but in
> the vast majority of cases you won't need these.
>
> On 31 October 2017 at 23:55, Daniel Kang via llvm-dev <llvm-
> dev at lists.llvm.org> wrote:
> > Dear LLVM developers,
> >
> >
> >
> > Hello,
> >
> > I’m trying to find a way of cross-compiling my c code against
> > Baremetal Cortex-M device (so target triple will be arm-none-eabi)
> > only using LLVM/Clang, and not using anything from GNU (ld or libc).
> >
> > I’m doing this to know which one of LLVM/clang and GCC produces
> > smaller flash image size because saving flash is a big deal in our projects.
> >
> >
> >
> > 1) When I just follow this -
> > https://clang.llvm.org/docs/CrossCompilation.html using LLVM/Clang
> > Prebuilt binary 5.0.0, I got this error,
> >
> > daniel at daniel-VirtualBox:~/clang/crosscompile$ make clean all
> >
> > rm -f main *.o *.map *.a
> >
> > "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin"/clang -static
> > --target=arm-none-eabi -mcpu=cortex-m0   -c -o main.o main.c
> >
> > "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin"/clang -static
> > --target=arm-none-eabi -mcpu=cortex-m0   -c -o a.o a.c
> >
> > "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin"/clang -static
> > --target=arm-none-eabi -mcpu=cortex-m0   -c -o reset.o reset.c
> >
> > "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin"/clang -v -static
> > --target=armv6m-none-eabi -o main main.o a.o reset.o
> >
> > clang version 5.0.0 (tags/RELEASE_500/final)
> >
> > Target: armv6m-none--eabi
> >
> > Thread model: single
> >
> > InstalledDir: clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin
> >
> > "clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin/ld.lld" main.o a.o
> > reset.o -Bstatic
> > -Lclang+llvm-5.0.0-linux-x86_64-ubuntu16.04/lib/clang/5.0.0/lib/bareme
> > -Lclang+tal
> > -lc -lm -lclang_rt.builtins-armv6m.a -o main
> >
> > clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin/ld.lld: error: unable to
> > clang+find
> > library -lc
> >
> > clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin/ld.lld: error: unable to
> > clang+find
> > library -lm
> >
> > clang+llvm-5.0.0-linux-x86_64-ubuntu16.04/bin/ld.lld: error: unable to
> > clang+find
> > library -lclang_rt.builtins-armv6m.a
> >
> > clang-5.0: error: ld.lld command failed with exit code 1 (use -v to
> > see
> > invocation)
> >
> > makefile:33: recipe for target 'main' failed
> >
> > make: *** [main] Error 1
> >
> >
> >
> > 2) To get clang_rt.builtins-armv6m.a or libc, libm, I tried to build
> > LLVM/Clang (compiler_rt) from LLVM/Clang source following this -
> > http://llvm.org/docs/HowToCrossCompileLLVM.html but I got this error,
> >
> > TARGET_TRIPLE=arm-none-eabi
> >
> > MYHOSTBIN=${HOME}/clang/source/build_x64/bin
> >
> > MYGNUARM_ROOT=${HOME}/opt/gcc-arm-none-eabi-6-2017-q2-update
> >
> > MYCFLAGS="--specs=nosys.specs -mcpu=cortex-m4 -mthumb
> > -mfloat-abi=softfp -mfpu=fpv4-sp-d16"
> >
> > cmake -G "Ninja" \
> >
> >                -DCMAKE_CROSSCOMPILING=True \
> >
> >                -DCMAKE_BUILD_TYPE=Release \
> >
> >                -DCMAKE_INSTALL_PREFIX=${HOME}/clang/toInstall \
> >
> >                -DLLVM_TABLEGEN=${MYHOSTBIN}/llvm-tblgen \
> >
> >                -DCLANG_TABLEGEN=${MYHOSTBIN}/clang-tblgen \
> >
> >                -DLLVM_DEFAULT_TARGET_TRIPLE=arm-none-eabi \
> >
> >                -DLLVM_TARGET_ARCH=ARM \
> >
> >                -DLLVM_TARGETS_TO_BUILD=ARM \
> >
> >                -DLLVM_ENABLE_LTO=Full \
> >
> >                -DCMAKE_C_FLAGS="${MYCFLAGS}" \
> >
> >                -DCMAKE_CXX_FLAGS="${MYCFLAGS}" \
> >
> >                -DCMAKE_SYSROOT=${MYGNUARM_ROOT} \
> >
> >                -DCMAKE_TOOLCHAIN_FILE=../toolchain.txt \
> >
> >
> > -DCMAKE_C_COMPILER=${MYGNUARM_ROOT}/bin/arm-none-eabi-gcc \
> >
> >
> > -DCMAKE_CXX_COMPILER=${MYGNUARM_ROOT}/bin/arm-none-eabi-g++ \
> >
> >
> > -DCMAKE_ASM_COMPILER=${MYGNUARM_ROOT}/bin/arm-none-eabi-as \
> >
> >                ../llvm
> >
> > …
> >
> > -- Looking for __atomic_load_8 in atomic
> >
> > -- Looking for __atomic_load_8 in atomic - not found
> >
> > CMake Error at cmake/modules/CheckAtomic.cmake:74 (message):
> >
> >   Host compiler appears to require libatomic, but cannot find it.
> >
> > Call Stack (most recent call first):
> >
> >   cmake/config-ix.cmake:350 (include)
> >
> >   CMakeLists.txt:585 (include)
> >
> >
> >
> > arm-none-eabi toolchain doesn’t contain libatomic, but I need to build
> > compiler_rt using this tool to get the runtime for ARM, so it’s confusing.
> > Or building compiler_rt using arm-none-eabi toolchain is completely
> > wrong idea in the first place.
> >
> >
> >
> > 3) Then I found this mail thread
> > http://lists.llvm.org/pipermail/llvm-dev/2017-August/116134.html which
> > seems very close to the thing I’m trying, but the mail thread is unfinished.
> >
> >
> >
> > So, my question is,
> >
> > Is there a way of cross-compiling c code against Baremetal Cortex-M
> > device only using LLVM/Clang, and not using anything from GNU (ld or libc)?
> >
> > If so, do I have to build LLVM/Clang source code to get
> > clang_rt.builtins-armv6m.a or libc, libm for ARM?
> >
> > If so, the method 2) or 3) above is the right approach? Or are there
> > any other instruction?
> >
> >
> >
> > Any advice would be a great help for me.
> >
> >
> >
> > Thank you
> >
> > Daniel
> >
> >
> >
> >
> >
> >
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> >
> >
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
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> >

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